You make a good point: There surely is a difference between someone like Donald Knuth writing a book that is relatively niche, and a best-seller. For one, I can imagine that a page of mathematics requires more expertise and time to write than a page of fiction.
However, there are dozens of free e-books on the web, written by respectable and knowledgeable professors, and this tells me that the only real cost involved is the effort of writing itself. The distribution and marketing cost for an e-book like 'Concrete mathematics' should be nearly zero. Unless my €80 goes directly to the authors, I'm wasting money. This is defendable for a physical book, but not for an e-book.
However, there are dozens of free e-books on the web, written by respectable and knowledgeable professors, and this tells me that the only real cost involved is the effort of writing itself. The distribution and marketing cost for an e-book like 'Concrete mathematics' should be nearly zero. Unless my €80 goes directly to the authors, I'm wasting money. This is defendable for a physical book, but not for an e-book.